Listen to Robert Emmerich introduce The Big Apple, a hit song from 1937. Music written by Bob and performed by Tommy Dorsey's Clambake Seven with Bob on piano. Lyrics written by Buddy Bernier and sung by Edythe Wright. Audio provided by Dorothy Emmerich.

"Sell when you can, not when you have to” is a market saying of uknown origin, cited in print since at least 1983. One should “sell when you can” (during a rising market, taking a profit) and “not when you have to” (in a falling market, taking a loss).

Google BooksBusiness Week
Issues 2772-2779
1983
Pg. 97:
A Salomon’s trader’s deft dealing in oil
A sign that hangs conspicuously on a wall inside Stanley Shopkorn’s office exhorts: “Sell when you can, not when you have to.”

11 July 1995, Washington (DC) Post, “Selling Below Market? Buffett and Tisch Quietly Show Some Real Forest Gumption” by Allan Sloan, pg. D3:
As they say on Wall Street, “Sell when you can, not when you have to.”

Google BooksBig Trends in Trading:
Strategies to master major market moves
By Price Headley
New York, NY: Wiley
2002
Pg. 137:
A famous maxim for bigger investors is “Sell when you can, not when you have to. “ This means that when there is a high demand for shares that the smart money owns, they will start selling out of positions, especially if news appears to be running the stock up.

StreetInsider.com
September 4, 2008 10:00 AM EDT
David Moenning’s Daily State of the Markets:
(...)
There is a saying in the investment community that applies here, “Sell when you can, not when you have to.” You see, when you run billions of dollars using big leverage to enhance your returns as well as your performance fees, the going can get tough when the market goes against you. And if things go badly enough, you may be forced to toss in the towel since most hedge funds have a preset self-liquidation clause once a certain level of losses are incurred.

TheStreetDividend.com: AIG Angst
By Tom Reese and Paul Rubillo 09/15/08 - 02:09 PM EDT
(...)
The “sell when you can, not when you have to” mantra couldn’t have been applied better than this particular situation we are reading about this morning.

1. “Never let your gains turn into losses.”
2. “Bulls make money, Bears make money, and Pigs get slaughtered. Don’t be a Pig.”
3. “Take something off of the table.” “Take the money and run.”
4. “No one ever got hurt taking a gain.”
5. “You’ve not made money until you’ve booked the profit.”
6. “Don’t be afraid to take a profit.”
7. “Ring the register!”
8. “Cut some of your winnings back and you will never regret it. Give back your winnings and you will never get over it.”
9. “If you’re paying taxes, you’re making money.”
10. “Sell when you can, not when you have to.”

Seeking AlphaHas Bernanke Failed?
by: David Moenning December 10, 2010
(...)
One of the oldest clichés in the hedge fund biz is that managers of big funds have to “sell when you can, not when you have to.”

The Big PictureArt Huprich’s Market Truisms and Axioms
By Barry Ritholtz - January 1st, 2011, 12:00PM
Raymond James’ P. Arthur Huprich published a terrific list of rules at year’s end. Other than commandment #1, they are in no particular order:
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• Sell when you can, not when you have to.